I remember I was only 10 years old when the horror movie Scream came out. I don't know how I talked my parents into letting me see it, but they did. To me, it wasn't a very scary movie, but I certainly didn't dress up as ghost face for Halloween that year.
Movies are given ratings for a reason, because the content may be inappropriate for children. Of course, there's always a little wiggle room in there and parents are welcome to use their discretion in deciding if their kid is mature enough to watch a movie they may "technically" be too young for. The thing is, though, not every kid matures at the same rate, and just like a scary movie is appropriate for some kids and not others, so are scary Halloween costumes.
We live in the age of epic superhero movies (team Thor!), questionable cartoons, and of course access to YouTube. Kids are exposed to some scary things, and maybe they love that feeling of being scared (like I did). Not all kids love that feeling, though, and not all parents want their kids to be scared, so sending your child to school in a scary Halloween costume is inappropriately exposing kids to something they may not be ready for and disrespectful to any parents who are trying desperately to avoid scary things.
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School is supposed to be a safe place for students. They already have to do lockdown drills and learn how to "run, fight, or hide" in the event of a shooting which makes going to school scary enough on a daily basis. They should not have to see your child with expert-level gruesome makeup that makes it look like they are a real-life zombie on top of the everyday fears.
Additionally, there could be students in your child's class that have fears or anxieties you know nothing about. One child may be experiencing night terrors on a regular basis, another may have suffered from a traumatic event that a scary costume could trigger, and another may just generally get scared easily.
The argument that there will be scary kids out trick-or-treating on Halloween night doesn't work here, either. For one, if a child that is easily scared does go trick-or-treating, chances are good they will have a parent with them to help them cope with anything that sparks fear. Second, kids don't have a choice whether or not to go to school, they have to show up and the class teacher should not be held responsible for explaining fiction vs reality to a scared student while they are also trying to corral kids during a class party.
Every parent is different and everyone has a right to choose to expose their children to sensitive things in their own time. Schools are required to send notices to parents when certain topics are covered in class because of this very issue. We need to respect the parent who does things a little differently even if we don't agree with their method. So, while you have every right to let your kid be as scary as they want on Halloween, you don't have the right to assume every parent feels the same way you do.
I get it, kids can have some really strong opinions about their costume and it may be easier for you to just let them wear what they want and ask forgiveness instead of permission. As parents, we have to choose our battles and maybe Halloween is just not one you want to choose. Unfortunately, you have to. Try to come up with a compromise for your child so they can still be a zombie but in a less scary way. Offer to make or buy them a second costume just for school and give them full reign to be as scary as they want during trick-or-treating. If nothing else, put your foot down and tell them they either follow the rules or they don't dress up at all. You're the parent, after all.
Try to remember the other kids when it comes to Halloween at school. Oftentimes we can be so focused on how we disagree with other parenting styles that we forget about the kids behind them. A parent who doesn't want a scary costume in school is not trying to tell you how to raise your kid, they're simply trying to protect their child and no mom or dad should be faulted for that.